Turbines are often provided with valves which control the flow of fluids such as gases or liquids (particularly steam) in various directions. The fluids are directed to turbine blades of the turbine to impart a rotational force or torque which is translated to a shaft. During turbine operation, it is necessary to vary the flow rate or pressure of entering fluid using control valves. For example, during start-up, the flow rate of fluid may be low so that turbine components can come to thermal equilibrium without incurring undo stresses. The flow rate is then increased to raise the turbine's rotational velocity. When such velocity or power output must be reduced, the flow rate of the motive fluid must be decreased. In certain turbine applications, changes in the rotational velocity require relatively small and precise adjustments. Further, fluid flow rate control valves should provide a tight seal to permit fluid flow to be shut off without leakage when the turbine is to be stopped.
Some presently available steam governor valves require applying an excessive amount of physical force to overcome the force of the steam pressure, and move the valve from a closed position to an open position. Other steam governor valves which have some means of balancing this large pressure force have excessive closed valve leakage, or poor low flow control. Presently used turbine governor valves are sized for small pressure losses at full load conditions and do not allow for small deviations in low fluid flow rates. It is therefore difficult to regulate fluid flow with precision at low or no-load operating conditions. It is especially difficult to regulate and maintain a low fluid flow rate when the inlet fluid is at a high pressure. Known steam turbine valves have generally poor control characteristics where, even at very small valve opening positions, the flow capacity increases much too quickly. Such valves deliver a relatively high fluid flow rate when even partially open. This makes precise turbine flow control especially low flow control difficult or impossible to achieve.
Many recognized valve designs have been tried in existing valve applications. Known "lantern-type" valves have double seats to allow pressure balancing. However, these valves require two typically beveled valve seats to both seal at the same time. Continued force fitting of the valve head into the seats leads to fatigue, and metal to metal wear of the valve seat, of control handles, and the threading mechanically linked to the valves. Where tight shut off during repeated cycles of valve openings and closings is required, valves with multiple ring seals have been used. U.S. Pat. Nos. 1,122,928 and 1,531,523 disclose piston type valves with multiple ring seals to seal off steam flow. The primary objective of these valves was a tight seal when steam flow was to be shut-off, and a large, fully opened flow capacity. Unfortunately, the valves are configured to allow only on/off operation, and as such are not applicable to precise flow metering required of turbine control valves.
A turbine governor valve which could 1) permit a range of predetermined fluid flow rates, including very small fluid flow rates, 2) provide complete shutoff, and 3) have low actuator force requirements would be highly advantageous.